1. Field of the Invention
The present invention relates generally to microwave phased-array antennas and more particularly to multiband phased-array antennas.
2. Description of the Related Art
Although the needs of many radar users can be satisfied with the generation of a single radar beam, other users require a plurality of radar beams which are each dedicated to a specific purpose. For example, major airports require radars that are directed to functions which can include medium-range air surveillance, long-range weather surveillance, airport surface detection, height-finding and traffic control. As a second example, naval shipboard environments require radars directed to functions that include long-range surveillance, navigation, weapons control, tracking and recognition and electronic warfare support measures (ESM).
Providing multiple antennas to handle such multiple tasks becomes especially difficult if the available antenna installation space is limited. This is particularly true in naval shipboard environments where the ship's superstructure is the preferred antenna location but there are numerous other demands for this space, e.g., bridge structures, ventilation and air conditioning structures and weapons mountings.
Because of its control of the phase of multiple radiating elements, a single phased-array antenna can simultaneously radiate and receive multiple radar beams. However, the unique requirements of the radar functions recited above typically dictate the simultaneous availability of radar beams which span multiple frequency bands. For example, long-range surveillance conventionally requires longer wavelengths, e.g., S band, precision-tracking and target-recognition radars generally operate most efficiently at shorter wavelengths, e.g., C band, and weapons control and doppler navigation are typically performed at still shorter wavelengths, e.g., X band and Ku band.
Because S band occupies the 2-4 GHz frequency region, C band occupies the 4-8 GHz frequency region and X band occupies the 8-12.5 GHz frequency region, radiation and reception of signals in all three bands requires a multiband, phased-array antenna with a bandwidth greater than two octaves. Such a single phased-array antenna with a bandwidth greater than two octaves could support multiple radar functions while being compatible with limited-space environments, e.g., shipboard.
A number of multiband radar antenna configurations have been proposed. For example, a structure of interlaced, contiguous waveguides was described in U.S. Pat. No. 3,623,111 which issued Nov. 23, 1971; an interleaved waveguide and dipole dual-band array antenna was described in U.S. Pat. No. 4,623,894 which issued Nov. 18, 1986 in the name of Kuan M. Lee, et al. and was assigned to Hughes Aircraft, the assignee of the present invention; and a coplanar dipole array antenna was disclosed in U.S. Pat. No. 5,087,922 which issued Feb. 11, 1992 in the name of Raymond Tang, et al. and was assigned to Hughes Aircraft, the assignee of the present invention.
Although these antenna configurations can radiate multiband antenna beams, the use of low frequency waveguides, e.g., S band (as proposed in U.S. Pat. No. 3,623,111), is preferably avoided because of their inherent bulk and the use of dipole antenna structures (as proposed in U.S. Pat. Nos. 4,623,894 and 5,087,922) is preferably avoided because of their inherent narrow-band performance.